1. Field of the Invention
The present invention relates generally to hydrogen utilizing devices, such as metal hydride batteries, and more specifically, relates to such devices having a hydrogen storage capacity for providing passive purification of a hydrogen stream which passively and automatically regulates and inhibits fluid flow of water vapor when the device is inactive.
2. Background Art
Metal hydride electrochemical fuel cells are in serious consideration as the next generation power source for providing storing and providing electric power to automobile and to other zero emission power storage and generation applications. Several innovative techniques have been described for storing and obtaining electrical power from electrochemical cells utilizing the combining reaction of hydrogen with oxygen to produce water and like combining reactions.
A major consideration for electrochemical cells utilizing hydrogen as reactant is the necessity of storing the hydrogen and providing and maintaining a clear, undiluted supply of hydrogen gas for use in the electrochemical reactions. For a detailed discussion of the background and considerations which enter into choosing components of an electrochemical battery system, and the requirements for such a system which utilizes a hydrogen storage capacity, reference is made to commonly assigned U.S. Pat. No. 5,532,074, the teachings of which are incorporated herein by reference.
A key consideration in avoiding material deterioration or decomposition of the components of the hydrogen storage system is the elimination of impurities, such as oxygen or water vapor, from the hydrogen gas stream delivered to the metal hydride storage material. Various methods have been proposed tending to inhibit or eliminate contact of oxygen or water vapor with the metal hydride hydrogen storage materials.
U.S. Pat. No. 5,128,219 describes an electrode protection mechanism for inhibiting contact of the metal hydride, hydrogen-storing negative electrode with the oxygen gas generated during the electrolytic reaction. In another method described in U.S. Pat. No. 5,250,368, the metal hydride storage material is isolated from the battery cell housing. However, even when isolated from each other, the electrochemical reaction in the battery cells produces sufficient water vapor that becomes entrained in the hydrogen gas stream to cause water vapor to reach the metal hydride and to release oxygen atoms when the hydrogen atoms are absorbed by the metal hydride material. The oxygen atoms tend to form oxides on the surface of the metal hydride material, causing "corrosion" of the surface and deterioration of the ability of the hydride material in sorbing hydrogen atoms. Thus, inhibiting contact of water vapor and/or oxygen with the metal hydride is an important consideration.
One solution to this difficulty has been proposed in U.S. Pat. No. 5,250,368. That proposal is to include certain elements in an in-line piping network between the hydride storage vessel and the electrochemical cell chamber or housing which are designed to purify and filter out the entrained water vapor and oxygen from the hydrogen stream passing through the in-line piping network or other gas communication means. A molecular sieve material is proposed which has a strong affinity for water vapor, but does not absorb hydrogen readily. During the charging of the battery, hydrogen gas is generated and flows from the battery charger through the in-line piping network to the metal hydride container. The molecular sieve material is contained within the in-line piping network and absorbs and removes the water vapor from the hydrogen stream down to a very low vapor pressure, on the order of 1 to 10 ppm before it reaches the hydrogen storage material.
During battery discharge, hydrogen gas leaves the metal hydride container and passes back through the in-line piping and the molecular sieve material. However, to evaporate the water in the molecular sieve material, electrical heating to about 250.degree. C. is required. At this temperature, the molecular sieve material cannot "hold" very much water and, therefore, rejects the water vapor back into the hydrogen stream, and rehumidifies the hydrogen stream before it (the H.sub.2 stream) enters the Ni/H.sub.2 battery cell. Since the purification mode used here requires external electrical energy for molecular sieve heating to "regenerate" the water vapor, it is considered an "active" purification process.
A passive water vapor filtering system, disclosed and claimed in commonly assigned U.S. Pat. No. 5,537,074 "filters" out the water vapor or oxygen impurities from the passing hydrogen gas stream. The water molecules are described as collecting on the surface of a film of filter material so that the impurities are not allowed to pass through to the metal hydride hydrogen storage material when the system is charging. During discharge, when the hydrogen gas is passing in the opposite direction, the water vapor "evaporates" and once again becomes entrained in the hydrogen gas stream.
Still another method of passive "filtering" of water vapor from a passing hydrogen gas stream is described in U.S. Pat. No. 4,343,770. One section of a communication means (filter unit) includes an adsorbent, selected from the group consisting of molecular sieves, alumina, charcoal and silica gel, to adsorb the water from a stream of hydrogen gas passing through the filter unit. When the hydrogen gas is discharged from the storage facility, it passes through the adsorbent thereby cleaning the adsorbent of water impurities and the hydrogen gas is then used where the impurities, such as water vapor, are immaterial, e.g., in a battery cell.
Passive purification of a hydrogen stream utilizing a water vapor absorbent material mixed together with a powdered metal hydride material is the subject matter of the related invention described in aforementioned U.S. patent application Ser. No. 08/673,104, filed Jul. 1, 1996, now U.S. Pat. No. 5,688,611. The present invention may be utilized with the passive purification system taught therein. Conversely, the present invention may be utilized with one hydrogen utilization system, as described above, or any other system which requires maintaining the hydrogen storage means separated from the hydrogen utilization system and for which a hydrogen stream without entrained impurities is desired. For example, the arrangement of the present invention is applicable in a hydrogen utilization system in which an extremely pure, i.e. vapor free, hydrogen stream is required, e.g., for use in an extra terrestrial atmosphere as will explained below.
Passive purification is most effective for enabling water vapor removal from a hydrogen stream during continuous operation of the fuel cell or device which consumes hydrogen. However, use of a fuel cell is continuous under conditions when it is either charging or discharging, without any downtime between the two operations when the hydrogen gas operations are dormant. During these dormant periods, the hydrogen gas pressure in the fuel cell chamber is equal or substantially equal to the hydrogen gas pressure in the metal hydride chamber, and essentially no hydrogen flows between the chambers.
The problems which develop from use of a system that is not continually either charging or discharging, i.e., when the system as a whole is in an equilibrium condition is that for an aqueous based utilization, such as a hydrogen fuel cell, water vapor develops within the enclosed chamber by evaporation from the fuel cell. A partial water vapor phase results within the hydrogen gas in the system and as a result of Brownian motion, the water vapor becomes dispersed throughout the hydrogen gas even when no hydrogen gas flow is present. For an arrangement without valves disposed between the two enclosed portions of a system, water vapor develops a uniform partial pressure that reaches the metal hydride/water absorbent mixture, and continually provides a partial pressure of water vapor over long dormant periods, until the system again begins hydrogen operation while either the charging or discharging the system.
During periods of equilibrium, when water vapor has permeated throughout the system, continual exposure of the surface of the metal hydride material to the water vapor causes the hydride material to react with the oxygen atoms in the water vapor to deteriorate the surface sorbing ability of the metal hydride. Even when used in a mixture with water sorbent materials, as taught in related U.S. Pat. No. 5,688,611, the continual exposure of the hydride surface to water vapor, coupled with the eventual water saturation of the water sorbent material by continual exposure to the water vapor, renders the system unable to continue optimal performance for hydrogen storage after a few cycles.
What is considered necessary to the industry is a system having an efficient and long-lasting metal hydride battery hydrogen storage means, which includes a means for inhibiting free travel of water vapor between the hydrogen storage means and the hydride battery cell, or other hydrogen utilization means during periods of inactivity of the system, when no hydrogen flow is present.